The applicant's U.S. Pat. No. 6,092,432 and International patent application WO 00/71909 are incorporated by reference and describe constant mesh transmissions employing hydraulic gear selectors used to engage free rotating gears on a first shaft to transmit drive to a fixed gear on an adjacent shaft. The present invention relates to developments in gear selectors suitable for the types of transmissions described and controlled in accordance with the general type arrangements disclosed in U.S. Pat. No. 6,092,432 and WO 00/71909.